Mechanisms for sediment fining in a side channel system

RCEM2017

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10

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_{Symposium on River,}

coastal and estuarine morphodynamics

Trento - Padova

September 15 – 22, 2017

Department of Civil, Environmental

and Mechanical Engineering

DIPARTIMENTO DI INGEGNERIA CIVILE, EDILE E AMBIENTALE

DEPARTMENT OF CIVIL, ARCHITECTURAL AND ENVIRONMENTAL ENGINEERING

RCEM2017

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RCEM 2017 - Back to Italy

The 10

th

Symposium on River, Coastal and

Estuarine Morphodynamics

Trento-Padova

15-22 September 2017

Book of Abstracts

Edited by:

Stefano Lanzoni, Marco Redolfi

and Guido Zolezzi

All rights reserved. No part of this book may be reproduced in any form, by photostat,

micro-form, retrieval system, or any other means, without prior written permission of the editors.

Proceedings of the event RCEM2017 10th Symposium on River, Coastal and Estuarine

Mor-phodynamics – Trento – Padova 15-22 September 2017/ editors Stefano Lanzoni, Marco Redolfi

and Guido Zolezzi

ISBN: 978-88-8443-752-5

© 2017 by RCEM2017 Organizing Commitee

RCEM 2017 - Back to Italy

The 10

Symposium on River, Coastal and Estuarine Morphodynamics

Organizing Committee Co-Chairs

Stefano Lanzoni

Guido Zolezzi

Local organizing committee

Adami Luca

Bertoldi Walter

Carniello Luca

Chinellato Sara

Crestani Elena

D’Alpaos Andrea

Defina Andrea

Ghinassi Massimiliano

Marani Marco

Redolfi Marco

Ruol Piero

Surian Nicola

Toffolon Marco

Tubino Marco

Viero Daniele Pietro

Welber Matilde

Zen Simone

1

Department of Civil, Environmental and Architectural Engineering, University of Padova,

Italy

2

Department of Civil Environmental and Mechanical Engineering, University of Trento, Italy

Department of Geosciences, University of Padova, Italy

252

Mechanisms for sediment fining in a side channel system

1_{University of Twente, Enschede, The Netherlands. r.p.vandenderen@utwente.nl} 2_{Ministry of Infrastructure and the Environment-Rijkswaterstaat, The Netherlands.}

1. Introduction

Side channels have been constructed in the Dutch river system to reduce flood risk and to increase the ecological value of the river. Some of these side channels show large aggradation and therefore require regular maintenance. Grain size measurements of the deposited sediment show that the bed of the side channel contains much finer sedi-ment (0.2-0.3 mm) than the bed of the main channel (1-2 mm). This suggest that sorting occurs at the bifurcation of the side channel which likely affects the equilibrium state and the time scale of the side channel development. The objective is to reproduce the morphodynamic devel-opment with a 2D numerical model that allows for sorting processes. The dimensions of the system are based on the Waal River in the Netherlands.

2. Method

We use a 2D Delft3D model and we compute the morpho-dynamic development of a side channel system as pre-sented in Figure 1. Initially the bed level of the side channel is higher than in the main channel which cor-responds with a side channel system just after construc-tion. The bed roughness in the side channel is smaller than in the main channel since bed forms are generally much smaller. The upstream discharge is given by a aver-age yearly hydrograph which includes one flood peak and at the downstream boundary a water level is prescribed which is based on the equilibrium water depth. We as-sume two sediment classes: one with a grain size which is similar to the sediment of the main channel bed and one with a grain size which similar to the deposited sediment in the side channel. The mixture is uniformly distributed over the system with the coarse sand volume fraction of 0.95. This corresponds with the volume fraction of the two mixtures which was found in the main channel in re-ality (Ten Brinke, 1997). The bed level and bed composi-tion changes are computed using the Hirano (1971) active layer model. The active layer thickness is assumed con-stant over time and equal to the bed form height (Blom, 2008). Initially, we compute the sediment transport for each sediment class with the Engelund and Hansen (1967)

Figure 1. The out line of the numerical model with a side channel which is shorter than the main channel.

relation, but we later extend this to other relations. The development of the side channel is related to, for ex-ample, the length difference between the channels and the presence of a bend upstream of the bifurcation. We there-fore test several cases in which the side channel varies in length, width and the location of the bifurcation with re-spect to the bend. A weir is often placed at the entrance of the Dutch side channel. This weir affects the morpho-dynamic changes in the side channel and likely the grain size in side channel. We therefore will include the effect of such a weir in our cases.

3. Discussion

Preliminary results show that the Engelund & Hansen re-lation for the sediment transport is unable to reproduce the fining of the bed in a side channel. Initially variations in the sediment volume fraction occur, but these seem to disappear in the morphological equilibrium. With En-gelund & Hansen both sediment classes react similarly to flow velocity changes. In other words, the nonlinear-ity of the sediment transport relation for both fractions is the same. This probably prevents sediment sorting to oc-cur and therefore several tests with other sediment trans-port relations which do include a nonlinearity as a func-tion of the grain size are tested to confirm this hypoth-esis. In addition, the preliminary results show that the transverse bed slope does influence the spatial distribu-tion of the grain size, because the parametrizadistribu-tion used is a function of the Shields stress. However, the effect of the transverse bed slope seems limited and might be un-derestimated. Future work will focus on using different sediment transport relations and including the effects of overbank flow.

Acknowledgments

This research is supported by the Netherlands Organisa-tion for Scientific Research (NWO), and which is partly funded by the Ministry of Economic Affairs. Grant num-ber P12-P14 (RiverCare Perspective Programme) project number 13516.

References

Blom, A. (2008). Different approaches to han-dling vertical and streamwise sorting in modeling river morphodynamics. Water Resour. Res., 44. doi:10.1029/2006WR005474.

Engelund, F. and Hansen, E. (1967). A monograph on sediment transport in alluvial streams. Technical re-port, Tekniskforlag, Copenhagen.

Hirano, M. (1971). River bed degradation with armour-ing. Trans. Jpn. Soc. Civ. Eng., 3:194–195.

Ten Brinke, W. (1997). De bodemsamenstelling van Waal en IJssel in de jaren 1966, 1976, 1984 en 1995. Tech-nical report, Rijkswaterstaat, Arnhem.